I'm pretty sure that is sun-fading damage. I saw it get progressively get worse over the last 10 years.
The latest picture above, I took that in July, and at that time, I took a couple of other pics from different angles.
Now, on anther shot I took maybe because of the different sun angle, while in person it looks blank, on camera zoomed in, I can sort of see smudges where the numbers should be.
That led me to go hog wild on the Gamma and Contrast settings and:

UPDATE EDIT, FEB 06 2021. I drove by this at night a couple of times and realized that the three faded signs must have had some kind of glass bead (reflective) paint for the numerals, because they showed up in the headlights. But a couple of days ago I noticed that these had been replaced, finally. Kind of odd regarding the new date codes though, one from 2015 and two signs from 2020 and they left the old I-25 sign in place. Oh, and I preferred the US shields on the old signs.
